CLOUD (from the same root, if not the same word, as "clod," a word common in various forms to Teutonic languages for a mass or lump; it is first applied in the usual sense in the late 13th century; the Anglo-Saxon clud is only used in the sense of "a mass of rock," wolcen being used for "cloud"), a mass of condensed vapour hanging in the air at some height from the earth.
Classification of Clouds. - The earliest serious attempt to name the varieties of cloud was made by J.B. Lamarck in 1801, but he only used French terms, and those were not always happily chosen. The field was therefore still clear when in 1803 Luke Howard published, in Tilloch's Philosophical Magazine, an entirely independent scheme in which the terms were all Latin, and were applied with such excellent judgment that his system remains as the broad basis of those in use to-day. He recognized three primary types of cloud - Cirrus, Cumulus and Stratus - and four derivative or compound forms, - Cirro-cumulus, Cirro-stratus, Cumulo-stratus and Cumulo-cirro-stratus or Nimbus.
His own definitions were: -
(1) Cirrus. - Parallel, flexuous or diverging fibres, extensible in any or all directions.
(2) Cumulus. - Convex or conical heaps, increasing upward from a horizontal base.
(3) Stratus. - A widely-extended continuous horizontal sheet, increasing from below.
(4) Cirro-cumulus. - Small, well-defined, roundish masses, in close horizontal arrangement.
(5) Cirro-stratus. - Horizontal or slightly inclined masses, attenuated towards a part or the whole of their circumferences, bent downward, or undulated, separate or in groups consisting of small clouds having these characters.
(6) Cumulo-stratus. - The cirro-stratus blended with the cumulus, and either appearing intermixed with the heaps of the latter or superadding a widespread structure to its base.
(7) Cumulo-cirro-stratus, or nimbus. - The rain-cloud: a cloud or system of clouds from which rain is falling. It is a horizontal sheet, above which the cirrus spreads, while the cumulus enters it laterally and from beneath.
This system was universally adopted, and apart from some ambiguity in the definitions of cumulo-stratus and nimbus, it was sufficiently detailed for many purposes, such as the general relations between clouds and the movements of the barometer. When, however, such questions as the mode of origin of particular forms of cloud came to be investigated, it was at once felt that Howard's classes were too wide, and something much more detailed was required. The result has been the promulgation from time to time of revised schemes, most of these being based on Howard's work, and differing from him by the introduction of new terms or of subdivisions of his types. Some of these new terms have come more or less into use, such as A. Poëy's pallium to signify a uniform sheet, but as a general rule the proposals were not accompanied by a clear enough exposition of their precise meaning for others to be quite sure of the author's intention. Other writers not appreciating how fully Howard's names had become established, boldly struck out on entirely new lines. The most important of these were probably those due respectively to (1) Poëy, published in the Annuaire de la société météorologique de France, 1865, (2) M. l'Abbé Maze, published in the Mémoires du congrès météorologique international, 1889, and (3) Frederic Gaster, Quart. Jour. R. Meteorological Society, 1893. In all of these Howard's terms are used, but the systems were much more elaborate, and the verbal descriptions sometimes difficult to follow.
In his book Cloudland (1894) Clement Ley published a novel system. He grouped all clouds under four heads, in accordance with the mode in which he believed them to be formed.
I. Clouds of Radiation.
Nebula Stillans Wet fog.
Nebula Pulverea Dust fog.
II. Clouds of Interfret.
Nubes Informis Scud.
Stratus Quietus Quiet cloud.
Stratus Lenticularis Lenticular cloud.
Stratus Maculosus Mackerel cloud.
Stratus Castellatus Turret cloud.
Stratus Precipitans Plane shower.
III. Clouds of Inversion.
Cumulus Heap cloud.
Cumulo-stratus Anvil cloud.
Cumulo-stratus Mammatus Tubercled anvil cloud.
Cumulo-nimbus Shower cloud.
Cumulo-nimbus Nivosus Snow shower.
Cumulo-nimbus Grandineus Hail shower.
Cumulo-nimbus Mammatus Festooned shower cloud.
Nimbus Rainfall cloud.
Nimbus nivosus Snowfall.
Nimbus grandineus Hailfall.
IV. Clouds of Inclination.
Nubes Fulgens Luminous cloud.
Cirrus Curl cloud.
Cirro-filum Gossamer cloud.
Cirro-velum Veil cloud.
Cirro-macula Speckle cloud.
Cirro-velum Mammatum.  Draped veil cloud.
It will be seen that Ley's scheme is really an amplification of Howard's. The term "Interfret" is defined as the interaction of horizontal currents of different velocities. Inversion is a synonym for vertical convection, and Inclination is used to imply that such clouds consist of sloping lines of falling ice particles.
While Ley had been finishing his work and seeing it through the press, H. Hildebrand-Hildebrandsson and R. Abercromby had devised another modification which differed from Howard's chiefly by the introduction of a new class, which they distinguished by the use of the prefix Alto. This scheme was formally adopted by the International Meteorological Conference held at Munich in 1891, and a committee was appointed to draw up an atlas showing the exact forms typical of each variety considered. Finally in August 1894 a small sub-committee consisting of Messrs H. Hildebrand-Hildebrandsson, A. Riggenbach-Burckhardt and Teisserenc de Bort was charged with the task of producing the atlas. Their task was completed in 1896, and meteorologists were at last supplied with a fairly detailed scheme, and one which was adequately illustrated, so that there could be no doubt of the authors' meaning. It is as follows: -
The International Classification.
(a) Separate or globular masses (most frequently seen in dry weather).
(b) Forms which are widely extended, or completely cover the sky (in wet weather).
A. Upper clouds, average altitude 9000 metres. 
a. 1. Cirrus.
b. 2. Cirro-stratus.
B. Intermediate clouds, between 3000 m. and 7000 m.
a. 3. Cirro-cumulus.
b. 5. Alto-stratus.
C. Lower clouds, 2000 m.
a. 6. Strato-cumulus.
b. 7. Nimbus.
D. Clouds of Diurnal Ascending Currents.
a. 8. Cumulus, apex 1800 m., base 1400 m.
b. 9. Cumulo-nimbus, apex 3000 m. to 8000 m., base 1400 m.
E. High Fogs, under 1000 m.
1. Cirrus (Ci.). - Detached clouds, delicate and fibrous-looking, taking the form of feathers, generally of a white colour, sometimes arranged in belts which cross a portion of the sky in great circles and by an effect of perspective, converge towards one or two points of the horizon (the Ci.-S. and the Ci.-Cu. often contribute to the formation of these belts). See Plate, fig. 1.
2. Cirro-stratus (Ci.-S.). - A thin, whitish sheet, at times completely covering the sky, and only giving it a whitish appearance (it is then sometimes called cirro-nebula), or at others presenting, more or less distinctly, a formation like a tangled web. This sheet often produces halos around the sun and Moon. See fig. 2.
3. Cirro-cumulus (Ci.-Cu.). - Small globular masses, or white flakes without shadows, or having very slight shadows, arranged in groups and often in lines. See fig. 3.
4. Alto-cumulus (A.-Cu.). - Largish globular masses, white or greyish, partially shaded, arranged in groups or lines, and often so closely packed that their edges appear confused. The detached masses are generally larger and more compact (changing to S.-Cu.) at the centre of the group; at the margin they form into finer flakes (changing to Ci.-Cu.). They often spread themselves out in lines in one or two directions. See fig. 4.
5. Alto-stratus (A.-S.). - A thick sheet of a grey or bluish colour, showing a brilliant patch in the neighbourhood of the Sun or Moon, and without causing halos, sometimes giving rise to coronae. This form goes through all the changes like Cirro-stratus, but according to measurements made at Upsala, its altitude is one-half as great. See fig. 5.
6. Strato-cumulus (S.-Cu.). - Large globular masses or rolls of dark cloud, frequently covering the whole sky, especially in winter, and occasionally giving it a wavy appearance. The layer is not, as a rule, very thick, and patches of blue sky are often seen through intervening spaces. All sorts of transitions between this form and Alto-cumulus are seen. It may be distinguished from nimbus by its globular or rolled appearance, and also because it does not bring rain. See fig. 6.
Issued as it has been with the authority of an international congress of specialists, this scheme has been generally accepted, and must be regarded as the orthodox system, and for the great majority of observations it is quite detailed enough. But it does not give universal satisfaction. Cirrus clouds, for instance, exhibit many forms, and these so diverse that they must be due to very different causes. Hence for the minuter study of cloud forms a more elaborate scheme is still needed.
Hence in 1896 H. H. Clayton of the Blue Hill observatory, Massachusetts, published in the Annals of the astronomical observatory of Harvard College a highly detailed scheme in which the International types and a number of subdivisions were grouped under four classes - stratiforms or sheet clouds; cumuliforms or woolpack clouds; flocciforms, including strato-cumulus, alto-cumulus and cirro-cumulus; and cirriforms or hairy clouds. The International terms are embodied and the special varieties are distinguished by the use of prefixes such as tracto-cirrus or cirrus bands, grano-cirro-cumulus or granular cirrus, etc.
Again in 1904 F. L. Obenbach of the Cleveland observatory devised a different system, published in the annual report, in which the International types are preserved, but each is subdivided into a number of species. In the absence of any atlas to define the precise meaning of the descriptions given, neither of these American schemes has come into general use.
Further proposals were put forward by A. W. Clayden in Cloud Studies (1905). His scheme accepts the whole of the International names which he regards as the cloud genera, and suggests specific Latin names for the chief varieties, accompanying the descriptions by photographs. The proposed scheme is as follows.
Cirrus Cirro-nebula Cirrus haze.
Cirro-filum Thread cirrus.
Cirrus Excelsus High Cirrus
Cirrus Ventosus Windy Cirrus
Cirrus Nebulosus Hazy Cirrus
Cirrus Caudatus Tailed Cirrus
Cirrus Vittatus Ribbon Cirrus
Cirrus Inconstans Change Cirrus
Cirrus Communis Common Cirrus
Cirro-stratus Communis Common Ci. S.
Nebulosus Hazy Ci. S.
Vittatus Ribbon Ci. S.
Cumulosus Flocculent Ci.-S.
Cirro-cumulus Cirro-macula Speckle cloud.
Nebulosus Hazy Ci. cu.
Alto-stratus maculosus Mackerel sky.
Alto-clouds Alto-cumulus castellatus Turret cloud.
Alto-cumulus glomeratus High ball cumulus.
Alto-cumulus stratiformis Flat alto-cum.
Stratus Stratus maculosus
Stratus maculosus radius Roll cloud.
Stratus maculosus lenticularis Fall cloud.
Cumulus Cumulus minor Small cumulus.
Cumulus major Large cumulus.
Cumulo-nimbus Storm cloud.
The term nimbus is to be applied to any cloud from which rain is falling, but if the true form of the cloud is visible the term should be used as a qualifying adjective. The prefix fracto- or the adjective fractus should be used when the cloud is undergoing disintegration or appears ragged or broken. Mammato- is used in the ordinary sense, and finally undatus or waved is to be added to the name of any cloud showing a wave-like or rippled structure.(A. W. C.)
 1 metre = 3.28 ft.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)